In recent years, a method was proposed in 1977 by F. F. Jobsis in which weak near-infrared rays (680-1300 nanometers) are irradiated from on the skin of the head through the skull and into the brain to measure changes in concentration of oxygenated hemoglobin (Oxy-Hb, HbO2) and changes in concentration of deoxygenated hemoglobin (Deoxy-Hb, Hb) in the blood at the brain surface (cerebral cortex) just inside the skull.
Since that time, research on the measurement of tissue oxygen concentration by means of this near-infrared spectroscopy (NIRs) method has progressed rapidly.
In general, the near-infrared spectroscopy method has the advantages that metabolism of individual tissue can be measured noninvasively from the surface of the body (noninvasiveness), it can furthermore be implemented by a simple and convenient apparatus (portability), and, in addition, it differs from imaging methods such as PET (positron emission CT), f-MRI (functional magnetic resonance imaging) in that it makes it possible to obtain real-time measurements of changes in tissue metabolism in the brain, muscles and the like over time (temporality); it has thus given rise to expectations of a wide range of application in uses such as brain function monitoring, evaluation of muscle rehabilitation in physical therapy, and exercise physiology.
Jobsis' previous method was an attempt at noninvasive brain oxygen monitoring, and an optical tomography method (optical CT) was devised, in which the brain was cross-sectioned in layers by straight-line light in an attempt to obtain accurate oxygen information from in the depths of the brain, (Shinohara, Y. et al., Optical CT imaging of hemoglobin oxygen-saturation using dual-wavelength time gate technique. Adv Exp Med Biol, 1993. 333: p. 43-6).
However, even if accurate location information could have been measured by the technique of optical CT, by the time light had passed through the skull to the brain surface and into the brain, it was absorbed, and so the method was of no practical use.
Accordingly, in 1991, the present inventor Kato devised and corroborated a new basic principle of NIRS imaging (near-infrared spectroscopy functional imaging) for determining location information by means of the location of a probe on the brain surface and the response to a measurement target.
In addition, the present inventor and his colleagues conducted light stimulus experiments in humans in which the brain was partially irradiated with near-infrared light, which showed, as a result, that localized brain function distribution can be monitored at the bedside, and proved that it is possible to create images of localized brain function using this method and a bedside noninvasive method for detecting local brain function (Sachio Takashima, Toshinori Kato, et al., “NIR Spectroscopy ni yoru kyokusho nouketsuryu hendou no kansatsu”, Shinshingaiji (sha) no iryou ryouiku ni kansuru sougouteki kenkyu no houkokusho [“Observation of variation in local brain blood flow by means of near-infrared spectroscopy”, in Comprehensive Research Report Concerning Medical Care for Children (People) with Disabilities (Japan Ministry of Health and Welfare), p. 179-181 (1992); Kato T, Kamei A, et al., “Human visual cortical function during photic stimulation monitoring by means of near-infrared spectroscopy”, J Cereb Blood Flow Metab. 13:516-520 (1993).
This basic principle of near-infrared spectroscopy brain functional imaging (NIRS imaging) is currently utilized in, for example, techniques for graphically displaying the functional topography (hemoglobin distribution, i.e., the display of variation in blood volume, reflecting brain activity, like a topographical map) of the brain surface in the frontal region, the occipital region and the like, and in pioneering techniques for obtaining information on brain activity. Subsequent techniques proposed for the graphical display of brain function include, for example, the inventions described in Japan unexamined patent publication nos. 2003-144437, 2003-75331, 2000-237194, H9-135825, 2002-177281 and 2003-10188.
The inventions proposed in these publications concern apparatus for measuring the interior of a living body by irradiating the living body with near-infrared light from a plurality of irradiation sites and detecting light transmitted through the living body at a plurality of detection sites; this is called Optical Topography (registered trademark), and changes in concentration of oxygenated hemoglobin and deoxygenated hemoglobin in the blood are calculated for each measuring point based on light intensity signals measured at a plurality of measuring points and displayed topographically.
It is furthermore utilized in pioneering techniques for obtaining information on brain activity, which occurs as rapidly as electrical activity.
For example, Gratton et al., by means of NIRS imaging, have detected a faint light that varies by means of electrical activity by adding 1-wavelength near-infrared light to a signal (occurring) approximately 100 ms before a brain blood flow response occurs from a stimulus consistent with an electrical response (Gratton G, Fantini S, Corballis P M, et al. Fast and localized event-related optical signals (EROS) in the human occipital cortex: comparisons with the visual evoked potential and fMRI. NeuroImage 6, 168-180, 1997).
Or, a technique has been proposed as a game or apparatus for displaying one's intent, by utilizing changes in cerebral blood flow and outputting them externally (International published patent application no. WO 00/074572 pamphlet, Yamamoto et al.).
Patent reference 1. Japan unexamined patent publication no. 2003-144437 Patent reference 2. Japan unexamined patent publication no. 2003-75331 Patent reference 3. Japan unexamined patent publication no. 2000-237194. Patent reference 4. Japan unexamined patent publication no. H9-135825 Patent reference 5. Japan unexamined patent publication no. 2002-177281 Patent reference 6. Japan unexamined patent publication no. 2003-10188 Patent reference 7. International published patent application no. WO 00/074572 pamphlet